Need for variable frequency control in DC-DC switching converters - Challenges and opportunities using digital implementation

Abstract

DC-DC converters play a vital role in efficient power management in various portable devices. However, the challenges remain in achieving fast dynamic performance, stable periodic behavior, and high efficiency over a wide operating range. In the context of fixed-frequency pulse-width modulation (PWM), a multi-loop current-mode control (CMC) scheme offers improved bandwidth and superior line regulation compared to a single-loop voltage-mode control (VMC) scheme. However, CMC suffers from sub-harmonic oscillations while operating with the steady-state duty ratio D > 0.5, which requires a compensating ramp for stabilization. A compensating ramp eventually degrades the closed-loop bandwidth, and a further increase in the ramp slope would tend to approach VMC; thus the benefits of CMC are lost with an increasing duty-ratio. On the other hand, variable-frequency CMC methods, such as hysteresis control and constant on/off-time control techniques do not suffer from inner current-loop stability problems, and also offer improved transient response and light-load efficiency. However, undesirable steady-state frequency variations complicate the input filter design and may eventually lead to EMI problems. While a digital platform is helpful for fast and precise frequency regulation using an all-digital-PLL, the choice of sampling frequency remains an important concern for implementing variable-frequency digital control. This paper highlights the challenges and scopes using variable-frequency digital control. The paper also presents event-driven digital control solutions and design insights for next-generation power management solutions.